1. Field of the Invention
The present invention generally relates to interior supports and a method for providing interior structural supports for a stack of circuit cards.
2. Description of the Related Art
Circuit cards or boards can be tightly stacked one on top of another inside computers, airplanes, spacecraft or electronic devices in general to conserve space and shorten the lead lengths between components on separate cards. The circuit cards are typically plugged into a backplane that holds them parallel to each other and facilitates communications between the individual cards. The cards' interior portions can sag under their own weight or deflect in response to vibrations at their resonant frequencies, and thus create stress on the circuit components and leads. The circuit card deflection increases as the card becomes larger because of greater sag and a lower resonant frequency. These problems complicate and increase the cost of design by forcing engineers to incorporate structural as well as functional considerations into their designs. For example, the heavier components may have to be placed along the periphery of the circuit cards.
An electronics module can be used to hold a number of circuit cards in close proximity and to electrically connect the cards. The electronics module is a metal box that is open at one end and has a backplane at the other end. Locking circuit card guides are installed on opposing side walls. A suitable card guide is disclosed in "Heat Sink/Dissipator Products and Thermal Management Guide," by IERC, Inc. of Burbank, Calif. sect. 7, p. 10, 1984. The locking guides include a pair of tracks that support the circuit cards and respective camshafts that can be rotated Go pinch down and hold respective cards. The circuit cards are slid into successive tracks and plugged into the backplane, which facilitates communication between the individual cards. When the cards are in place, the guides are locked by rotating the camshafts such that each circuit card is supported on three sides by the guides and backplane, and separated from the adjacent cards by about 1.25 cm-1.9 cm (by 1/2 to 3/4). Other card guides use different mechanisms to clasp the circuit cards' edges. Card guides improve the cards' peripheral support, but the lack of interior support can cause stress in the electrical components and complicate the card design.
FIG. 1 is a sectional view of a conventional approach that uses stand-offs 21 to provide interior structural support between circuit cards 22 to reduce sagging and deflection. The circuit cards 22 are plugged into a backplane 24, which electrically connects the cards and holds them parallel to each other. One type of stand-off 21 includes an internally threaded sleeve 26 and two screws 28 that can be threaded into the sleeve's opposite ends. The circuit cards are fabricated with complimentary hole patterns such that in each card half of the holes are collinear with holes in the card immediately above it and the remaining half are collinear with the holes in the card immediately below it. Therefore at each successive card the holes are offset from those in the previous card in order to thread the screws into the sleeves. At a given level in a stack of circuit cards, the stand-offs for the next card are placed over the appropriate holes in the current card and the screws 28 are threaded up through the current card to secure the stand-offs. Thereafter, the current card is placed over the previous card's stand-offs and the screws 28 are threaded down through the current card to secure it to the stack.
Another type of stand-off is a sleeve that has an internally threaded (female) end and an externally threaded (male) end. At each successive level, the current card is placed over the supports for the previous cards, the male ends of which extend through the current card, and the female ends of the next set of sleeves are threaded onto the protruding male threads. The sleeves supporting the first and last cards are fastened to these cards by screws and nuts, respectively. In either case the interior supports, typically one or two at each level, effectively partition the circuit cards to reduce the sag and increase the resonant frequency, thereby reducing the cards' deflection.
Although stand-offs can provide adequate interior support, the assembly and disassembly of a stack of circuit cards using stand-offs can be very difficult and time consuming. The stand-offs are assembled by hand one at a time on each successive card in the stack, starting from the bottom. To remove a circuit card for repair or replacement, all of the cards and stand-offs subsequent the defective card must be disassembled one at a time, starting from the top card. After the defective card has been replaced, each successive card must be put back and their respective stand-offs reassembled. Another drawback to the sue of stand-offs is the possibility that a sleeve or screw may be dropped, causing an electrical short or requiring extensive disassembly to retrieve it.
Typically the physical structure that supports and houses the circuit cards is as small as possible and is tightly packed with circuit cards and other electrical components. In some applications it may be difficult for a technician to access the circuit cards to attach the standoffs, and it might be necessary to remove the entire housing or selected panels to gain access. From a packaging and structural integrity perspective, a unitary structure that is not disassembled and reassembled periodically is preferable. For example, the four side panels and backplane in the electronics module form a unitary structure. The circuit cards are slid through the respective card guides, plugged into the backplane and locked down. A cover can then be attached over the open end of the guide to encase the circuit cards. Assembling and disassembling the stand-offs in this type of structure while efficiently using all of the available space would be very difficult and time consuming. Therefore in conventional practice, standoffs are not used in conjunction with card guides. Thus, either the peripheral support provided by the card guide or the interior support provided by stand-offs are achieved, but not both.